US 2014/0275854 A1 describes a biometric monitoring device which is used to determine a user's heart rate by using a heartbeat waveform sensor and a motion detecting sensor. In some embodiments, the device collects collecting concurrent output data from the heartbeat waveform sensor and output data from the motion detecting sensor, detects a periodic component of the output data from the motion detecting sensor, and uses the periodic component of the output data from the motion detecting sensor to remove a corresponding periodic component from the output data from the heartbeat waveform sensor. From this result, the device may determine and present the user's heart rate.
Photoplethysmography (PPG) has been developed to measure variations in blood volume in human tissue and thereby detecting a pulse signal of the heartbeat.
Typically, in PPG monitoring, light emitting diodes (LEDs) with wavelengths between 520 nm (green) and 850 nm (infrared) are used in combination with a photodiode. Transmission type PPG measurements use typically wavelength ranges (e.g. 650-850 nm) longer than reflection type measurements (e.g. using wavelengths in the range 520-570 nm).
Theoretically, the reflectance measurement can be taken at any skin surface. Moreover, the path length of reflectance in tissue is much shorter than that of transmittance. Both in reflectance and transmittance measurements, the signal-to-noise ratio of the heartbeat is based on the amount of absorption of the blood.
Conventionally, transmittance measurements are in general more robust measurements in comparison to reflectance measurements. Therefore, less power of light is needed as for reflectance measurements. There are two reasons for this:
1. The optical path length through the blooded tissue in transmittance is larger than in reflectance, so the signal-to-noise ratio of the measured variation in blood during a heart pulse is larger in transmittance.
2. For transmittance measurements, mostly longer wavelengths are used, that penetrate deeper in the skin and have less interaction with scatter particles in the tissue.
Optical losses in PPG sensing are due to absorbance, reflectance and scattering in the sensor part and in the human tissue. In particular, optical losses take place between the light delivery system and the skin by reflectance losses when light penetrates from one media into the other and by surface scattering onto the skin.
Optical skin properties and skin morphology are used to simulate losses due to skin reflectance. The total reflectance at different angles of incidence has been simulated for different melanin fractions (skin type II and III on the Fitzpatrick scale) and the results are illustrated in FIG. 1 for a single wavelength 520 nm). It can be concluded that due to Fresnel losses on the skin surface, at high angle of incidence (>60°) the reflectance at the skin rapidly increases in respect to normal angle of incidence (0°).
A major performance indicator of PPG sensors is the so called modulation, which is defined as the ratio of the AC over DC signal (see FIG. 2). Here AC is the signal one wants to measure (the pulsating arterial blood fraction; i.e. the change in blood volume) and the DC part is the unwanted background signal. Although the use of the term DC indicates a signal of 0 Hz, it is actually a low frequency disturbance caused by, for example, leakage light shunted from source to detector without passing through tissue (static), variation of the previous term caused by motion (dynamic) and light reaching the detector reflected from any tissue/matter other than pulsating arterial blood (e.g. venous blood, fat, bone, water, cell membranes, etc).
Currently, in any PPG sensor the DC part is much larger than the AC part, therefore minimizing DC and/or maximizing AC is one of the major challenges in the design of a good PPG sensor.
The PPG signal reflects the blood movement in the vessels. The quality of the PPG signal depends amongst others on blood flow rate, skin morphology and skin temperature. Also the optical losses in the system determine the quality of the PPG signal.